Various types of wireless networks have been developed and deployed including 3G, WLAN, WiMAX, LTE and LTE Advanced. User connectivity and network performance can be improved using vertical handover techniques which involve switching between available networks in heterogeneous environments. In this respect, recently there has been an increased interest in the integration of cellular and WLAN systems. In this study, the integrated heterogeneous wireless systems are modelled using two-stage open queuing systems. The proposed analytical model builds a framework for acceptable levels of QoS in heterogeneous environments. This paper gives the detailed analysis of the integrated cellular/WLAN systems based on the deployment of guard channels and buffering at base station for highly mobile users in the cellular systems. Numerical results are obtained using the exact spectral expansion solution approach. These results are then analysed in terms of operational spaces and are shown to be useful for vertical handover decision management.